Fossilization is a rare occurrence, because natural materials tend to be recycled. In order for an organism to be fossilized, the remains need to be covered by sediment as soon as possible. There are different types of fossils, and fossilization processes:
This process consists of literally turning an organism into stone. The organism gets covered by sediment soon after death, or after the initial decaying process. The degree in which the remains are decayed when covered, determines the later details of the fossil. Some fossils only consist of skeletal remains or teeth; other fossils contain traces of skin, feathers or even soft tissues. Once covered with sediment, these layers slowly compact to rock, after which the chemicals in the remains are slowly replaced with hard minerals.
Mould, cast and trace fossils
If percolating water dissolves the remains of an organism, and thereby leaves a hole, this is called a mould fossil. If this hole is filled with more minerals, it is called a cast fossil. If the burial of the organism was rapid, then chances are that even impressions of soft tissues remain. Trace fossils are the remains of track ways, burrows, footprints, eggs and shells, nests and droppings. The latter, called coprolites can give insight in the feeding behavior of animals, and can therefore be of great importance.
Smaller animals, insects, spiders and small lizards, can be trapped in resin (amber), which oozes from trees. These fossils can be found in sand- or mudstones.
These are regular patterns in rocks, which are produced by natural occurring processes. They can easily be mistaken for real fossils. These fossils can be formed by naturally formed fissures in the rock that get filled up by percolating minerals. Other types of pseudofossils are kidney ore, round shapes in iron ore, and 'moss agates', which look like plant leaves.
A term used for any living species which closely resembles a species known from fossils, i.e., as if the fossil had "come to life". This may sometimes be a species known only from fossils until living representatives were discovered (the most famous example of this is the coelacanth fish (Latimeria chalumnae)). Other "living fossils" are the nut clams (Ennucula superba), Lingula anatina, an inarticulate brachiopod, and the horseshoe crab (Limulus polyphemus).
Additional information see Fossils and the geological timescale
The study of fossils is called paleontology.